While Ive seen, and handled thousands and thousands of penises, the number that were actually erect were relatively small, maybe in the range of 200-300. Except for phimosis cases (thats where the foreskin is too tight during erection, causing pain) all the rest were "unplanned erections". all that said, I would guesstimate (most patients would probably have a problem if I brought out a tape measure) that the largest erect one Ive seen is in the 9 inch range. It was a 15 year old boy I gave a physical to work at the town pool last year. He surprised me because one second he was flaccid, and as soon as I started a testicular exam, he became erect in a matter of seconds. He was extremely apologetic, and embarassed. I told him not to worry, that it happens all the time (which it doesnt), but to warn me next time, as he almost caught me flush in the face. He laughed at that.
By the way, the smallest erection I ever saw was .75 inches. That one I measured, as he was a patient specifically because of that problem. I referred him to a plastic surgeon